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The teenager's death on Tuesday sparked disturbances across Turkey

As tension rose, Mr Erdogan again appealed for calm, urging his opponents to express their opinions peacefully in local elections later this month.

"Whatever issue you do have, solve them in polls on March 30," he said.

Earlier, crowds holding pictures of the teenager gathered first outside a house of worship in Istanbul where his body lay.

By Selin GiritBBC News, Istanbul

"Berkin Elvan is immortal" - this was the most uttered slogan during the 15-year-old's funeral.

Many people could not yet come to terms with the fact that this boy had actually lost his struggle to stay alive.

"269 days," one man said to me. "He was in a coma for 269 days. Nothing came out of the investigation to why he got injured. Of course, we are very angry."

At the funeral, there was not a visible police presence. One man told me: "They can't come here. If they do, hell will break loose."

How police react to the mourners and protesters out on the streets - not only in Istanbul but also across several cities in Turkey - might be crucial in how these protests evolve.

His coffin - draped in red and covered in flowers - was then carried through the streets to the leafy hill-top cemetery, surrounded by huge crowds.

As his coffin was lowered into the ground, Berkin Elvan's distraught mother, Gulsum Elvan, cried out: "What am I to do now? They've taken my everything."

Mourners shouted "Berkin's murderers are the AKP police", referring to Mr Erdogan's ruling Justice and Development Party (AKP).

"The rage of mothers will suffocate the killers," screamed others.

Later, protesters in Istanbul threw fireworks into police lines while hundreds of people - including bystanders caught up in the melee, sheltered in a shopping centre and a hotel lobby as police fired tear gas and pepper spray.

The boy's father, Sami Elvan, told the BBC's Newshour programme that his son had become a "child of the people".

Asked how he would like him to be remembered, he said: "I want him to be known as a child killed by the state. He was just an innocent boy killed when he went out to buy bread."

Correspondents say Berkin Elvan became a symbol of the heavy-handed tactics used by police to rein in the biggest demonstrations against the prime minister.

The media response in Turkey

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The photo of a crying mother, her head thrown back in anguish, is splashed across the front pages of many of Turkey's privately-owned daily newspapers.

"The grief of all of us" headlines the centrist daily Vatan while the leftist Karsi proclaims: "We all know the murderer".

Other papers quote the words of Berkin's mother. "[PM] Erdogan has taken my son", reads the headlines of Evrensel, Aydinlik and Yurt newspapers.

The front page of Turkey's mass-circulation Hurriyet simply states: "We are mourning". In a commentary, writer Taha Akyol wonders: "If even the life of a youngster can't be our 'common value', what values do we have left to keep us together?"

And Mehves Evin in Milliyet says that "Today, holding bread in our hands, we will commemorate Berkin and all innocent children who were sacrificed to state violence."

But for some of the pro-government papers - such as Yeni Akit and Sabah - it is business as usual, with pictures of Erdogan's election campaign filling the front pages.

Renewed pressure

His death brought the toll from last year's unrest to at least eight, including one policeman.

His mother had challenged Mr Erdogan, who praised police "heroism" during the protests.

"It's not God who took my son away but prime minister Erdogan," Mrs Elvan told reporters on Tuesday.

The renewed unrest is likely to add to pressure on Mr Erdogan, whose government has been rocked by an escalating corruption scandal ahead of elections that could decide his fate.

"How many young people have to die for Erdogan to resign? My only wish is for this fascism to end without spilling more blood," said retired worker Atilla Izmirlioglu.

Mr Erdogan has vowed to step down if the AKP, in power since 2002, loses in the local elections. The polls are seen as a key test of his popularity after last year's unrest and recent corruption scandals.

The sons of three former cabinet ministers were arrested and accused of corruption in December last year, while an audio recording surfaced that appeared to have caught Mr Erdogan talking to his son, Bilal, about hiding millions of euros in cash.

He said last month that the recording, allegedly tapped and then posted on social media, was fabricated and part of a "treacherous attack" by US-based Islamic cleric Fethullah Gulen.